Before resetting search index, make sure no search crawls are in progress - Stop all search crawls, disable continuous crawl - otherwise, index reset may get stuck! After index reset, You must run full crawl to enable search results!

PowerShell script to reset index in SharePoint 2013:
At times, you may have to do it with PowerShell for reasons, such as:

When you try to reset search index from Web UI, you get timeouts error on large index